I'll take a swing at this. I assume that for your three wires you want a ground, a + 12v, and a + 24v. If true then wire it like this:

Put the black wire on the negative terminal of battery A; this will be your ground. Connect the positive terminal on battery A to the negative terminal on battery B. Connect the wire (red or orange) that is supposed to deliver 12 volts to either the poitive terminal of battery A or the negative terminal of battery B (since they are connected, they are elecitrically the same. Might be easier to keep straight if you connect it to the positive terminal of battery A). Connect the wire that is supposed to deliever 24 v to the positive terminal on battery B. I tried to put a diagram in but this sucked up the blanks.

Hope this helps. Remember this is free advice; if you fry it I won't buy it! If you want to talk about it, call my cell 831-359-1049.

Buzzzfish